When commercial broadcast television and radio stations were first being built, a means of implementing a reliable, high power broadcast transmitting antenna was required. There were many antenna designs that were put forward during that time. Included in these many designs, (most of which emerged in the 1950's and 1960's), was a Transverse Electromagnetic Mode, (TEM), coaxial slot-style collinear antenna design. This highly-reliable design is rugged and extremely power tolerant. It is coaxial, meaning that it consists of two metallic conductors with different diameters. The larger conductor is hollow so that the smaller conductor can be placed inside of the outer one. The conductors are arranged vertically and vertically-oriented slots are cut or machined, longitudinally through the wall of the outer conductor. Usually, these slots are at least one-half of a free-space wavelength long, and are placed longitudinally at specific intervals along the length of the outer cylindrical conductor, such that the relative magnitudes and phases of the electromagnetic waves from each individual radiating structure along the array will form the desired far-field characteristics, such as pattern and elevation array gain.
Since this coaxial assembly forms a TEM-Mode coaxial transmission line, TEM-Mode electromagnetic fields from the transmitter connected to the inner and outer conductors of the antenna, alternate at the channel frequency and are set up in the space between the outside surface of the inner conductor, (the conductor that is placed inside of the outer conductor pipe, tube or cylinder), and the inside surface of the outer conductor. A structure called a coupler provides a means of coupling to the electric and/or magnetic TEM-Mode fields inside of the coaxial pylon antenna array. This coupler is electrically connected to one side of each slot in the array, and will impress a voltage or electric potential difference across the slot opening. Since the slots represent the absence of a conductor, and if the coaxial pipe is oriented vertically, (with the slots also oriented vertically), the alternating voltage across each slot will give rise to an electric field, whose direction points horizontally from one longitudinal slot edge to the other. The magnitude of the electric field will depend on how strongly the coupling structures at each slot couple to the TEM-Mode system of fields inside of the coaxial pylon. Again, if the coaxial pylon and hence, the slots are oriented vertically, then the voltage giving rise to the electric fields across the slots will be oriented horizontally. This oscillating horizontal electric field from each of the slots in the array will add together in the far-field, (depending on the relative magnitudes and phases from each of the slots, or electromagnetic radiating centers in the array), forming a horizontally-polarized, propagating electromagnetic wave.
If these radiating slots in a vertically-oriented cylindrical outer conductor are analyzed, it is seen that the same coupling structures that excite a voltage across the slots cut in the conducting cylinder or pipe will excite horizontal currents that will “loop” around the outer pipe's outer circumferential surface. These slot-voltage induced horizontally-oriented circumferential currents will then give rise to oscillating magnetic fields, who's North-South and then South-North direction is positioned vertically. Maxwell's Equations describe that this vertically-oriented oscillating magnetic field will give rise to an oscillating electric field, whose orientation is horizontal, (just as seen from the electric fields across the slots in the array). This is an example of a horizontally-polarized, slotted pylon antenna.
The radiation moments excited by the slot-excited circumferential currents that originate and terminate at each of the two longitudinal edges of the slots will exhibit radiation resistance that varies over frequency according to a number of factors, including the channel or radio frequency, pylon diameter, (hence, electrical length of the circumferential current path around the cylinder), the number of slots per level along the length of the pylon as well as other factors. This will greatly affect the reflected power, (or V.S.W.R.), at the input terminals of the coaxial pylon, as well as the V.S.W.R. frequency bandwidth, as well as the azimuth and elevation far-field electromagnetic radiation pattern frequency sensitivity.